This invention relates to the art of electrochemical cells, and more particularly to a new and improved anode coating for alkali metal-halogen cells.
The recent growth in small-sized electronic devices has required electrochemical cells having high energy density and current delivery capability. As a result of this need alkali metal halogen and alkali metal oxyhalide cells have been developed wherein the anode typically is lithium and the electrolyte is either solid or liquid. In the development of such cells it has been found advantageous to coat the operative anode surface with organic material. In the alkali metal/oxyhalide solid electrolyte cells the anode coating advantageously results in a greater utilization of the surface of the lithium anode by the cathode material of the cell and a reduction in cell impedance. In the liquid electrolyte cells the anode coating advantageously inhibits the formation of compounds on the anode surface which otherwise would cause passivation which gives rise to a voltage delay after storage at elevated temperatures.
One method of coating lithium anodes is applying a solution of the organic material in solvent to the anode surface by painting or brushing. Examples of this method are found in U.S. Pat. Nos. 3,957,533, 3,993,501, 4,296,185 and 4,608,322. This method obviously requires careful work by the applicator, is time consuming, requires exposure of the lithium anode during drying, and may require certain control procedures in handling the solvent. Another method is forming a sheet of the organic material such as by hot pressing and then applying the preformed sheet of the organic material to the anode surface, often in conjunction with an adhesive. Examples of this method are found in U.S. Pat. Nos. 4,182,798 and 4,398,346. This method requires use of a greater amount of the organic material since the sheet is entirely of such material, and the hot pressing procedure has energy requirements as well as handling requirements including selection of materials for the platens which will not adhere to the hot pressed organic material.
In coating lithium anodes with a pre-formed film, it has been determined desirable according to the present invention that the film have some flexibility. When the film is made from organic donor material using solvents alone, it is particularly difficult to retain flexibility of the film under dry room conditions. Also, flexibility becomes particularly important when the pre-formed film is to be applied to a lithium anode surface which is not flat but instead has surface formations, i.e. grooves, for the purpose of increasing the available anode surface for operative contact with the cathode material. Such an anode surface is illustrated, for example, in U.S. Pat. No. 4,201,708.